In traditional seismic prospecting an explosive seismic wave energy source is positioned in a relatively shallow shot hole at the surface of the earth and an acoustic energy impulse of some preselected wave shape and frequency content is generated thereby. The acoustic energy generated by the source propagates through the multiple layers of material comprising the subterranean portion of the earth in the region being prospected. Since the speed of sound is generally different in the various geological layers beneath the surface of the earth, refractions, reflections, and diffractions of the acoustic energy waves occur at the boundaries of each layer. At the surface of the earth the returning acoustic energy waves are sensed by a plurality of seismic detectors. These detectors produce electrical signals which characterize the magnitude and frequency content of the acoustic energy waves detected. These electrical signals are typically recorded on magnetic tape and are used later in conjunction with a large number of similar recordings to generate a seismic "section", i.e. a display of a cross-section of subterranean geological structure. These seismic "sections" can be very useful in the search for underground mineral and petroleum deposits.
The above-described traditional technique for seismic prospecting has certain serious drawbacks. One problem is the expense associated with the preparation of a large number of widely spaced shot holes. Another problem is the significant surface and air noise connected with such explosions. Furthermore, there is an element of danger connected therewith.
U.S. Pat. No. 2,688,124 issued to Doty, et al, on Aug. 31, 1954, disclosed a very useful alternative approach for seismic prospecting. The Doty approach employs a vibrating seismic energy source instead of the traditional explosive source. These vibrating seismic signals are varied in frequency in a unique non-repetitive manner over a period of time. The received seismic signals are then correlated with a counterpart of the transmitted signal in a manner which results in the production of an accurate measurement of the seismic signal travel time between the vibrator source and the seismic signal detector. The Doty technique was a very significant contribution to the seismic art, and is currently in wide-spread use. The method is commonly referred to as the VIBROSEIS technique (VIBROSEIS is a service mark of the Continental Oil Company).
The Applicants have observed a serious practical difficulty connected with the implementation of the above-described technique. In particular, the dynamic range of the recording medium is often substantially expended in the recording of the direct waves and ground roll signals which are also generated by the vibrator source. Thus, the desired reflected seismic signals tend to be restricted to only a small portion of the dynamic range of the recording medium, since they are much smaller in magnitude than the direct waves and ground roll. The result can be a drowning out or masking of the desired signals by these extraneous signals which do not contribute to the mapping of subsurface structure. Since all recording media have a finite dynamic range, it would be very desirable to eliminate the ground roll and direct wave signals from the output signals of the seismic detectors prior to the recording thereof--thus allowing substantially the entire dynamic range to be used for recording of significant data.